Lithium batteries, particularly lithium secondary batteries, have features such as high energy density and long service life, and are widely used as power supplies for electrical appliances such as a video camera, or for portable electronic equipment such as laptop computers and mobile telephones. Recently, application thereof to large-sized batteries that are mounted in electric vehicles (EV), hybrid electric vehicles (HEV) and the like has been anticipated.
A lithium secondary battery is a secondary battery having a structure in which, at the time of charging, lithium slips out in the form of ions from a positive electrode and then migrates to a negative electrode where the ions are stored, and at the time of discharging, lithium ions on the contrary return from the negative electrode to the positive electrode, and the higher energy density of the lithium secondary battery is known to be attributable to the potential of the positive electrode material.
As the lithium transition metal oxide that can be used as a positive electrode active material for lithium secondary batteries, lithium transition metal oxides having a layered structure, such as LiCoO2, LiNiO2 and LiMnO2, and lithium transition metal oxides having a manganese-based spinel structure (Fd-3m), such as LiMn2O4 and LiNi0.5Mn1.5O4, are known.
Among them, spinel type lithium transition metal oxides are inexpensive in terms of raw material cost, non-toxic and highly safe, and therefore, attention is paid to spinel type lithium transition metal oxides as a positive electrode active material for large-sized batteries such as electric vehicles (EV) or hybrid electric vehicles (HEV). Furthermore, while excellent power output characteristics are particularly required in batteries for EV's and HEV's, in this regard, spinel type lithium transition metal oxides which are capable of three-dimensional insertion and extraction of Li ions are particularly excellent in the power output characteristics, as compared with lithium transition metal oxides such as LiCoO2 having a layered structure.
In regard to such spinel type lithium transition metal oxides, it is known that if impurities, particularly sulfides (sulfate radicals) such as SO4 or alkali metals other than Li, are present in large amounts at the surface of the oxide particles, there are various adverse effects on the battery characteristics, such as a decrease in the cycle characteristics or storage characteristics.
Particularly, when a spinel type lithium transition metal oxide is produced using electrolytic manganese dioxide as a manganese raw material, because electrolytic manganese dioxide is produced in a manganese sulfate electrolyte bath, a relatively large amount of sulfides such as SO4 come into existence at the surface of particles. Therefore, addressing this problem is important.
Thus, methods of removing impurities at the surface of particles by washing with water a spinel type lithium transition metal oxide obtained through calcination, have been hitherto employed (see, for example, Patent Documents 1 to 3).